


between the sea and the sky (the world holds its breath)

by A_Thousand_Skies (cywscross)



Category: Dr. STONE (Anime), Dr. STONE (Manga)
Genre: Alternate Universe - Demons, Alternate Universe - Supernatural Elements, Background Character Death, Conspiracy, Demon & Human Interactions, Demon Senkuu, Demon Yuzuriha, Demon/Human Relationships, Don't copy to another site, F/M, Human Ryuusui, Hybrid Francois, Hybrid Taiju, M/M, Other Additional Tags to Be Added, Private Investigators, Scars
Language: English
Status: In-Progress
Published: 2019-11-14
Updated: 2019-11-14
Packaged: 2021-02-01 07:00:49
Rating: Teen And Up Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,020
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21428596
Author URL: https://archiveofourown.org/users/cywscross/pseuds/A_Thousand_Skies
Summary: Their first mistake was going after a man like Nanami Ryuusui. Their second was not making sure it was theironlymistake, because now Ryuusui is certainly going to guarantee that it will be their last. He just needs a little help.There are people on both sides who would very much like to see the world embroiled in the Old Wars once more, but there are just as many who don't. For Senkuu, living a (mostly) normal life with his two best friends is enough for him, and he won't stand for anyone who threatens that. Not even for the Demon Kings.~~Or, the human/demon AU literally nobody asked for.
Relationships: Ishigami Senkuu/Nanami Ryuusui, Ogawa Yuzuriha/Ooki Taiju, Other Relationship Tags to Be Added
Comments: 9
Kudos: 138





	between the sea and the sky (the world holds its breath)

Nanami Ryuusui has only ever cared about exactly three things in his life - his freedom, his ships, and Francois.

The fire that ravages the Nanami estate one summer night spreads too quickly and burns too hot, and by the end of it all, once Ryuusui wakes up in the hospital covered in burns, he is told that all his model ships and vessel prototypes were reduced to blackened wood and ash, his childhood home is in ruins, and while his most faithful companion survived (_survived after dragging Ryuusui out of the house_), they are also deep in a coma that nobody thinks they’ll wake up from.

Oh, and his entire family is dead too, but Ryuusui never cared about them when they were alive; he isn’t about to start now just because they’re no longer around to judge him.

He also refuses to accept his new lot in life. He checks himself out of the hospital as soon as he can walk without falling flat on his face, and he doesn’t allow the doctors to give up on Francois. He shoves money at them, and at every first-class medical research facility he can reach, and he sets them all on his butler’s case. He doesn’t let himself believe that Francois won’t wake up one day.

And then he goes hunting, because everyone else might say it was just an _unfortunate accident_, and Ryuusui himself isn’t well-versed in magic, but he _knows_ that there was something strange about that fire.

(Spread too quickly and burned too hot, and even as Ryuusui had screamed from the fire eating away at his flesh and choked on the smoke filling his lungs, he’d swear on everything he holds dear that he’d heard the echo of a laugh and the flash of ash-white hair out of the corner of his eye.)

He takes it to the police first, but the best he gets from them are pitying looks and polite refusals to reopen the case because he has no real evidence of foul play. He tries sifting through the remains of the estate - his now, what’s left of it - but he can find nothing out of the ordinary. He even reaches out to his family’s contacts - those who might have heard about someone plotting against the Nanami family, or might have connections to the demon world that he does not - but suddenly, nobody seems to want anything to do with him anymore, and he gets more doors - metaphorical or otherwise - slammed in his face than he ever did back when he’d acted out in ways that every noble disapproved of.

The Nanami name, he quickly comes to realize, no longer holds even half as much status and influence in the eyes of the other families. Their stocks are in tatters, their political and societal weight near-nonexistent, and while these things can be built back up, Ryuusui has never had much interest in any of it, and it would take too long to go this route just for some measly crumbs of information that he might not even get once he’s networked and strong-armed his way back to the top.

He’s the only one left, and he’s never been anything close to the perfect heir.

But he’s also _Nanami Ryuusui_, and giving up has never been in his nature. He lost almost everything he cares about overnight, but that doesn’t mean he can’t get them back.

* * *

He should’ve paid more attention to Francois’s demon heritage, or even just demons in general. As a modern-day rule of thumb, humans and demons do not interact or even mingle. Even in a single city, the population is usually divided - humans living and working on one side, demons on the other. Demons scorn humans for their weakness; humans detest demons for their power. But humans also outnumber demons a hundred to one, while one demon of average power could probably kill a couple dozen armed humans on their own if they really wanted to. The Blood Wars of old had been a dark and brutal chapter in history, raging for over 3700 years until even hatred wasn’t enough motivation for either side to continue fighting. In the end, neither species had come out on top. so a truce had been struck, treaties had been proposed, and laws had been written and rewritten and rewritten again. And over the past few hundred years since then, the world had slowly settled into a grudgingly tolerant existence with each other.

Nowadays, tensions have eased even further. There are still people on both sides who cling to the belief that they’re better than the other. The Nanami family had been one. Most of the rich and powerful human elite consider demons little better than animals, only fit to serve them, and Ryuusui’s heard rumours of some demons - the really powerful, never-seen-in-public ones - who want to restart the Blood Wars.

But it also isn’t so uncommon anymore for regular human citizens to cross paths with regular demon citizens. Cities are segregated, but they also take the same trains and shop at the same malls, and in some places, establishments that serve humans and demons have begun to open, and a handful of schools have even started accepting students of both species.

It’s progress, however slow.

Ryuusui himself has never seen much reason for animosity against anyone - humans or demons - who have done nothing to him personally. Demons have never factored into his life, he’s never had any bad experiences with them, and one of his points of pride has always been to ignore approximately everything his family has ever tried to teach him, including demon discrimination.

The demon world was not one Ryuusui ever felt the need to step into, not even after he met Francois.

He met Francois on a normal afternoon, found them while he was at his private shipyard, tinkering with one of his earlier-built vessels. He’d stopped for lunch and found half his food gone and a small skinny figure darting out the back door. One wild chase around the shipyard later, Ryuusui had finally cornered them, and it had been clear they had demon blood in them - the riot of bedraggled blond curls haloing their head had been pretty human-normal but the hostile pink eyes full of false bravado that glared back up at him definitely weren’t.

They hadn’t attacked though, and later, Ryuusui would find out that they couldn’t. Because Francois was half-and-half, one of the rare hybrids that came from a human-demon union.

Too demon for human society, too human for demon society. Rejected by both worlds, the real dirty secret both sides kept under the floorboards. Francois had none of a regular demon’s naturally enhanced strength and speed and senses, but their eyes were also a dead giveaway of their demon heritage, too bright for even coloured contacts to hide. And apparently, full-blooded demons carry a certain scent that Francois simply didn’t have, so demons could sniff them out of a crowd too.

In the end, Ryuusui had taken them in. He’d never met a demon before that point, and he’d figured it wouldn’t hurt to have an extra pair of hands around. In return, he’d feed and clothe and house them. It was a good deal, and Francois had been suspicious of course, but they’d been desperate enough to take him up on it anyway.

Eight years down the road and Ryuusui’s never regretted it once. All demons are born with a talent, a unique set of powers ingrained in their DNA, along with a title they grow up _knowing_. Francois tells him early on that they’re a Household Demon, which comes with everything from excellent cooking skills to the ability to fold shirts perfectly to gaining a sixth sense of the needs of whatever household they decide to take on. And that household doesn’t necessarily have to be a house either. Ryuusui is living proof of that.

All in all, Francois makes the perfect butler, and others might’ve jeered at them for the subservient role, but it’s what Francois is comfortable with, and Ryuusui has always paid them for their unparalleled services and made sure they knew they were always free to leave.

Francois never did, not even after they earned enough to make their own way in the world.

They’re friends more than anything else after eight years of companionship, and when Ryuusui wakes to Francois in the hospital bed beside his, silent and small beneath starch white sheets, he knows he’ll never stop until whoever was responsible for the fire pays for it with their _life_.

* * *

In the end, someone from the military - or navy, Ryuusui supposes, on shore leave and visiting his police friends - takes pity on him as well but also proves at least moderately useful by giving him a business card.

It reads simply:

_YUSENTAI AGENCY_   
_Private Investigators_   
_132 Winter Moon Lane_

_We’ll take any <strike>exciting</strike> case!_

No, Ryuusui isn’t seeing things. The _‘exciting’_ really is crossed out.

Purportedly run by demons, it’s probably the only investigative firm entrenched in the demon world that also caters to humans.

Ryuusui will take it. The very next day, he visits Francois, makes sure his fellow humans aren’t getting any ideas about accidentally-on-purpose killing a half-demon in their care, and then leaves again to make the five-hour drive to the little town that this Yusentai Agency is supposed to be based out of.

He finds it, tucked between other shops on a street in the demon part of town. The sign is simple but elegantly carved and beautifully painted, hanging above the door. It seems to span two floors, and the interior is dark, just like every other store on the block because a lot of demon establishments still run on a nocturnal schedule, but the signboard out front declares in flowery script:

> _We’re Open!  
Walk-In Customers Welcome!_

Below that, there’s a bunch of other additional messages written in three different hands:

> _We don’t take boring cases so don’t be boring._
> 
> _We’re human-friendly demons! Don’t be scared!_
> 
> _If you hear an explosion, it’s okay. Unless the building collapses, feel free to come in anyway!_
> 
> _Hey did anyone pick up those talismans from Nightshade’s?_
> 
> _I did! I put them in the back._
> 
> _STOP BUYING SO MUCH YARN._

And so on and so forth. Ryuusui stares for a long moment, and maybe he’s clutching at straws, wishful thinking getting the better of him after months of hitting dead-ends every way he turns, but there’s something about these random light-hearted back-and-forth exchanges that manages to unravel some of the stress that’s been knotted in his shoulders and sitting heavy in his gut since the fire.

Because it’s true that Ryuusui has no particular bias against demons, but he also knows that Francois was treated badly by them, even if they’ve never really delved into their past.

Demons don’t like humans. Humans don’t like demons. Those are facts of life as much as water is wet and snow is cold. There are exceptions, but one can never be _certain_.

Ryuusui takes a deep breath, stomping down hard on the anxiety creeping up on him, absently rubbing at the burn scar covering part of his right hand. Then he makes himself stop because he keeps scratching the wound open without realizing.

He gives himself a mental shake and strides determinedly up to the innocuous door.

When it comes down to it, it doesn’t matter whether or not _these_ demons will treat a human like him fairly. They’re his best lead, and so long as they help him, he’ll pay them as much as they want.

The Nanami fortune is good for something at least.

He pulls open the door and steps inside. The floorboards creak under his feet. The distant chime of a bell echoes through the interior as his eyes adjust.

A girl - maybe only a little younger than him - with brown hair and brown eyes and a smiling face appears. She seems normal enough, except for the fact that Ryuusui hadn’t heard any footsteps at all.

“Oh, a customer!” The girl beams. “Welcome to Yusentai Agency!”

**Author's Note:**

> Do I know what I'm doing? No. Am I doing it anyway? Unfortunately.


End file.
